1. Field of the Invention
The present invention relates to a write-once type optical data recording medium filled up with suitable fillers, having a recording layer suitable for a melting and diffusion recording system.
2. Description of the Related Art
Conventionally, there is known an optical data recording medium (recording single-plate) comprising a substrate including a predetermined irregular signal pattern, and a recording layer formed thereon composed of a recording material for a melting and diffusion recording system. Data is written on the optical data recording medium by causing a thermal deformation such by melting, diffusion and the like of the recording layer by irradiating a highly concentrated beam, e.g., a laser beam, on the recording film thereof. This type of optical data recording medium often has an air sandwich structure, wherein two recording single-plates, each having a recording layer are bonded through an air layer with the respective recording layers facing inward. Note that the same structure may be used when a protective plate-shaped member having substantially the same shape and size as the recording single-plate, but not provided with a signal pattern, and a recording layer is bonded on the side of the recording layer of a recording single-plate.
This air sandwich structure is roughly classified into a sealed type wherein the air layer is sealed and a venting type wherein the air layer is communicated with the outside air, and each type includes a reinforcement type wherein two recording single-plates have a reinforcing plate held therebetween or a single-plate and a plate shaped member have a reinforcing plate held therebetween, and a cover glass type wherein the outer periphery of a recording medium having signal patterns formed on both front and back surfaces thereof is covered a cover glass through an air layer, and the like, depending on the combination of members used for forming the air layer.
The optical data recording medium having the former sealed type air sandwich structure, however, has serious drawbacks in that the recording single-plate and other plate shaped member become warped and thus a normal record reproducing operation becomes impossible, and in the worst case, and a crack will be generated in the substrate or the recording layer or a bonded portion is peeled off because the pressure in the air layer is greatly varied by changes in the weather conditions, such as temperature, atmospheric pressure and the like.
On the other hand, the above problems do not occur in the optical data recording medium having the venting type air sandwich structure because the pressure in the air layer is always equal to the outside air pressure, but this structure has problems in that the recording layer is easily corroded and the life of the medium is shortened because foreign substances, such as water and dust, are easily deposited on the recording layer.
As shown, for example, in Japanese Patent Publications Nos. 57-66540 and 57-130243, an improved bonding type optical data recording medium is known wherein a protective layer, including an adhesive for binding, is disposed on a recording layer, and two substrates are bonded with the protective layer held therebetween.
This optical data recording medium, however, has a problem in that, when the recording layer of the recording medium is used as a recording layer for a melting and diffusion system and a recording is effected by forming pits in the recording layer by heat-melting the layer, satisfactory pits cannot be formed. More specifically, while the heat transfer coefficient of the protective layer including the adhesive must be low enough and the protective layer must be wet resistant to the molten body of the heat-melted recording layer, as far as possible, the above protective layer has a problem in that it has a strong close adhesion force (good wetting property) to the recording layer, and thus the molten recording material will not diffuse well, the formation of satisfactory pits is prevented, and the sensitivity is greatly reduced. As a result, this optical data recording medium cannot be practically used.